This invention relates to liquid metal ion sources, and, more particularly, to enhancement of the wetting of an evaporation element of the ion source by an ion source alloy.
Liquid metal ion sources provide high current densities of metallic ions from a source having a small virtual source size. Such high brightness and small source size are required when the ion beam is to be focused with a high resolution of, for example, less than one micrometer spot size, and utilized in applications such as fabrication of semiconductor microcircuits. The high current density and small virtual source size are achieved by emitting the ions from a substrate having a sharp point, such as the point of a needle. In one such approach, a needle is covered with a layer of liquid ion source metal, and a cusp in the liquid metal at the point of the needle is created by application of an electrostatic field. This fine cusp then becomes the emitting source for evaporation of the ions. As the ions are emitted from the source, more liquid metal must flow from a reservoir down the needle to the cusp to replenish that evaporated.
For this type of high brightness ion source to operate properly, the ion source metal must wet the needle to ensure a smooth flow of metal from the reservoir to the cusp. If the ion source metal does not wet the needle, or wets the needle incompletely, the source metal alloy may form balls or lumps along the surface of the needle, thereby interfering with the metal flow, preventing the formation of the cusp, and increasing the apparent source size, with the result that the emitted ion beam cannot be properly focused.
In some instances, ion source alloys naturally wet the needle, which is typically made of common materials such as steel, nickel, tungsten, rhenium, metal carbides, etc. In other cases, however, there is no suitable needle that is wet by a desirable ion source alloy. As an example, lead-gold-antimony alloys and lead-gold-arsenic alloys used as ion sources for antimony and arsenic do not readily wet these needle substrate materials at the preferred operating temperature of about 200.degree. C.-300.degree. C. There exists a need for an approach for enhancing the wetting of ion evaporation elements such as ion source substrate needles.
One proposed approach to enhancing wettability is simply to increase the ion source substrate operating temperature at which the ions are evaporated, so that the surface energy of the ion source alloy is reduced and wetting is more easily achieved. However, this solution is not workable, since increasing the temperature also increases the evaporation rate of volatile ionic species such as antimony and arsenic. In the alloys discussed above as examples, increasing the temperature above the preferred operating temperature preferentially evaporates the metalloid species, so that the composition of the ion beam varies with time. This variation is not acceptable in operations such as ion implantation in semiconductors, since performance of the semiconductor circuit depends upon the exact composition of the implanted ionic species. It has also been found that various mechanical techiques such as melting the ion source alloy at the junction between the reservoir and the needle substrate, or directly dipping a needle into the molten alloy, do not lead to substantial wetting.
Consequently, there continues to exist a need for a technique for enhancing the wettability of ion source alloys on evaporation elements such as needles and reservoirs. Any such technique should not result in a substantially increased melting point of the ion source alloy, which could result in preferential evaporation of the volatile components. Further, the approach should not result in a significant variation of the evaporation rate and composition of the ion beam with time. The present invention fulfills this need, and further provides related advantages.